VeraPDF seems like the best option when it comes to openness, being free, and having Linux support (really it uses Java, so it’s just platform agnostic).
I’ve never used it, and it only has a Windows installer, but you might still be able to make use of it with Wine/Bottles/etc. Also, it’s free, so testing it out shouldn’t cost anything but time.
VeraPDF seems like the best option when it comes to openness, being free, and having Linux support (really it uses Java, so it’s just platform agnostic).
A second option might be this one:
https://pac.pdf-accessibility.org/en
I’ve never used it, and it only has a Windows installer, but you might still be able to make use of it with Wine/Bottles/etc. Also, it’s free, so testing it out shouldn’t cost anything but time.